                1. Field of the Invention        
The present invention relates to a rain tyre equipped with a tread pattern particularly configured to improve the holding of the tyre on a wet roadbed. Even more particularly, the tread pattern is configured to improve the holding of the tyre when cornering on a wet roadbed.                2. Description of the Related Art        
It is known that a vehicle driven on a road with a wet roadbed is subject, in particular conditions, to the phenomenon of aquaplaning. This phenomenon involves a substantial loss of adherence of the tyre, with obvious serious consequences for the safety of the vehicle which, in fact, proves to be out of control in this phase.
The phenomenon of aquaplaning is substantially attributable to the lifting up of the tyre from the surface of the road as a result of the hydrostatic pressure exerted by the film of water present on the roadbed. This pressure arises from the impact of the water against the advancing tyre, and is directly linked to the speed of the vehicle: as soon as this pressure equals the ground contact pressure of the tyre, the latter is in fact partially lifted from the ground, thus reducing its bearing surface on the road, and consequently losing adherence to the terrain.
In order to limit the occurrence of the phenomenon, one or more grooves are typically provided on the tread band of tyres, generally extending over the circumferential extent of the tyre.
The main function of these grooves is that of defining, on the front of the advancing tyre, one or more passages for the water. In this way, at least a fraction of the water affected by the advance of the tyre does not impact directly on its surface, reducing the overall hydrostatic pressure exerted on the tyre.
Besides the above-mentioned circumferential grooves, there are, typically, transverse grooves present on the tread pattern, extending from one of its lateral shoulders towards an equatorial zone of the tyre. A principal function of these grooves is that of permitting the rapid discharge of the water towards the outside of the zone of contact between tyre and roadbed, known in the field and identified hereinafter as “footprint area”.
A further important factor in the dynamics of the triggering of the phenomenon of aquaplaning is provided by the mutual connections between the various grooves. Such connections, in fact, although on the one hand they increase the volume of voids, on the other hand, actually in the footprint area, they cause turbulence and eddies to occur at the zones where different grooves meet, which slow down the flow of water through the grooves, locally increasing the hydrostatic pressure on the tyre.
It is pointed out that, in the present context, an end or a portion of groove is generally defined as “blind”, when it is not in connection with other grooves adjacent thereto. In particular, two grooves are defined as “not in connection” with one another when the passage of water from one to the other is substantially impeded. It should be noted, however, that the aforesaid definition does not exclude the possibility that the portion of groove may be open at the outer lateral edge of the tread band. Moreover, an end or a portion of groove may, in general, prove to be only partially blind, for example only on the axially outer side or on the axially inner side. In this case, the end or the portion of groove will prove to be not in connection with grooves adjacent thereto on the axially outer side or, respectively, on the axially inner side.
In the present context, it is stated that by “module” of the tread pattern there is to be understood a portion of tread pattern repeated, just the same, in succession along the entire circumferential extent of the tread band itself. The modules, however, while maintaining the same pattern configuration, have generally different circumferential lengths. This technical stratagem becomes necessary in order to limit the noise and vibrations of the rolling tyre, by distributing the characteristic frequencies over a wider spectrum.
More precisely, in this description and in the following claims, a “long pitch” module and a “short pitch” module are defined, in which the long pitch module has a circumferential dimension at least 20% greater than the corresponding circumferential dimension of the short pitch module.
The long pitch and short pitch modules are generally disposed circumferentially in succession, one after the other, in a sequence of long pitch and short pitch modules suitably designed to obtain the desired effect of noise limitation. It is further possible to provide modules formed of a combination of short pitch and long pitch sub-modules, thus increasing the number of possible combinations. By way of example, by combining a short pitch sub-module with a long pitch sub-module it is possible to obtain four different kinds of modules: a first formed by two short pitch sub-modules, a second formed by one short pitch sub-module and one long pitch sub-module, a third formed by one long pitch sub-module and one short pitch sub-module, and a fourth formed by two long pitch sub-modules.
For this reason, in the present context, the term “short pitch module” or “long pitch module” is to be regarded as referring, in a completely analogous manner, both to the module in the strict sense, when this is not formed by sub-modules or, in the opposite case, to the individual sub-modules which form it.
A further parameter used in the technical field of reference for the quantitative evaluation of the presence of grooves in the tread pattern is the “void-solid” ratio, a non-dimensional number defined as the ratio between the tread surface area having grooves and notches, and therefore not bearing on the roadbed, and the total surface area of the tread. This parameter may be evaluated both on the entire tread and on a specific portion thereof.
EP 1752314, among its different exemplary embodiments, shows a tyre having a tread band on which is defined a tread pattern comprising a main circumferential groove, extending along an equatorial plane of the tyre, and also a plurality of transverse grooves extending from the respective opposed shoulders of the tyre towards the main circumferential groove, wherein the axially inner end section of each transverse groove is blind, when in the footprint area.
U.S. Pat. No. 5,924,464 shows a tyre on the tread band of which is defined a tread pattern comprising a plurality of main grooves extending circumferentially around the tyre, and also a plurality of transverse grooves extending between the main grooves, wherein each transverse groove comprises a portion having an enlarged cross-section, connected to the adjacent main grooves via respective connecting passages which are partly or completely reclosable, when in the footprint area.